Measuring vitamin C in fruit using titration | 14-16 years

In this practical activity, students work in groups to analyse different samples of fresh fruits and juices, using titration to determine the amount of vitamin C present. They then compile a full set of class results, before planning an investigation to explore how vitamin C levels are affected by different conditions.

Learning objectives

Students will be able to:

• Carry out a simple titration.
• Find out differences in the amounts of vitamin C levels present in fruit.
• Find out how vitamin C levels are affected by different conditions.

Sequence of activities

Before the session

Prior to this lesson, suggest that students bring a piece of fresh fruit or juice sample for testing for vitamin C. It is best to bring the juice in the original packaging.

Introduction

1. Invite suggestions about where vitamin C comes from, together with ideas about how much vitamin C is present in the source suggested.
2. Organise suggestions in a primitive table – ’lots’, ‘some’ and ’little vitamin C’.
3. Introduce the practical, to test different fruits and juices for their vitamin C levels, and the learning objectives. Explain that the test will confirm or change some of the suggestions made.

Testing samples to determine vitamin C levels

1. Organise students into groups of three or four.
2. Give each student a copy of ’Testing for vitamin C’
3. Assign each group to a set of equipment.
4. Demonstrate the technique for testing the samples.
5. Ensure there is a minimum of four samples to test in each group.
6. Check for overlap – the same fruit or juice may be tested more than once by different groups, giving multiple results for reliability.
7. Circulate and supervise the groups as they extract juice from fresh fruit and test the juice samples.
8. Collate the results.

Reviewing the results

When the groups have completed their tests, review their results.

In a class discussion, review all the results and:

• Use any duplicate results to help students understand about the reliability of test results.
• Refer to juice packaging materials to give background information.

Investigating conditions affecting vitamin C levels

1. Introduce the next activity in which students plan an investigation to explore how vitamin C levels are affected by different conditions.
2. Give each group a different investigation to plan, using the basic test as a starting point. Suggested investigations include:
   • How does temperature affect the amount of vitamin C?
   • How does light affect the amount of vitamin C?
   • How does packaging and storage surroundings affect the amount of vitamin C?
   • How does a ‘slow release’ vitamin C tablet work?
   • Do fruits of different varieties (eg mandarin oranges, navel, clementines, satsumas, etc.) contain different amounts of vitamin C?
   • Does frozen, concentrated orange juice contain as much vitamin C as fresh juice?
   • Which fruit contains the most vitamin C per 100 g? Make a thorough comparison of as wide a range of fruit varieties as possible.
3. Agree criteria for peer assessment of investigation plans.
4. Give each group access to information sources and presentation materials.
5. Circulate and support as students:
   • Plan their investigations.
   • Prepare a presentation of their plan.
   • Elect a spokesperson to feedback to the class.

Plenary

In a plenary:

• Hear each presentation.
• Apply the agreed criteria.
• Give feedback based on the quality of the proposed investigations and their scientific content.
• Ensure that each student makes a written record of the planned investigation.

The investigations could be carried out during a later session or in an after school club.

Feedback

Give written feedback that reinforces the good points about the investigations, giving suggestions for improvements.

Commentary

Working together on a simple titration gives a sound basis from which groups work in teams to plan an investigation. The suggested investigations permit differentiation by task and outcome.

Agreeing the criteria and then assessing each other’s investigation plans stimulates students to perform well. The presentations also give an opportunity to evaluate activities across the class, adding coherence and depth to scientific understanding of the issue.

Teacher feedback after the practical work can point out the key scientific features for the investigation plans. Feedback on the presentations will reinforce the peer assessments. Feedback on written work supports development of scientific thinking.

Practical notes

For the full procedure, see the ‘Testing for vitamin C’ sheet.

Apparatus

• Eye protection
• Droppers or plastic pipettes, one for each juice
• Test tubes, one per test, allowing six per group
• Test tube rack
• 10 cm x 10 cm white card for background
• Juice extractor and beakers for juice if fresh fruit is used
• Materials for creating a presentation such as a large sheet of paper and marker pens, OHTs and OHT pens, networked computer or writeable CD ROM and IWB access

Chemicals

• Fruit juices to test, with packaging if the fruit is not fresh
• Water, about 100 cm³ per group
• Access to a solution of 0.05 mol dm^-3 iodine (HARMFUL) in potassium iodide (see ‘Health, safety and technical notes’, note 4), prepared using:
  • Iodine, 1.27 g
  • Potassium iodide, 1.5 g
  • Distilled water, 100 cm³
• Access to 0.1% starch solution
• Access to a 1 mg cm^-3 vitamin C solution, prepared using:
  • Water, 100 cm³
  • Vitamin C tablet, 100 mg, to be dissolved in the water

Health, safety and technical notes

1. Read our standard health and safety guidance.
2. Wear eye protection.
3. It is the responsibility of the teacher to carry out an appropriate risk assessment.
4. See CLEAPSS Recipe Sheet RB050 for preparing iodine solutions.

Answers

Example results table

Fruit juice / vitamin tablet	Number of drops needed to react with all the iodine
1000 mg vitamin C tablet in 100 cm3	15
Lemon juice	30
Lime	35
Orange juice from 1 day open carton	40
Kiwi	15
Melon	20
Mango	30

Note that the kiwi fruit contains the most vitamin C.

Questions

1. See results.
2. See results.
3. To ensure the end-point is not missed.
4. See results and packaging.
5. The amount of oxidation – vitamin C is lost by oxidation when the juice is exposed to air.